1. Field of the Invention
The present invention relates to an active matrix semiconductor device integrally formed, on a common substrate, with a matrix circuit serving as an image display section or photoelectric conversion section and a driver circuit (drive circuit) to drive the matrix circuit.
2. Description of the Conventional Art
Recently attentions are drawn to semiconductor devices having a circuit formed by a plurality of thin film transistors (hereinafter abbreviated as TFTs) on a glass substrate. Such semiconductor devices involves image display devices, such as liquid crystal displays and EL (electroluminescence) displays.
In particular active matrix display devices formed with pixel matrix and driver circuits on one common substrate are occupying steady positions for next generation displays taking place of CRT, by virtue of their capabilities to display images with precision in comparison with conventional passive type display devices.
The realization of such active matrix display devices have been contributed by the existence of semiconductor called polysilicon (polycrystalline silicon or fine crystalline silicon). Before utilizing polysilicon, the passive type display devices have utilized amorphous silicon for TFT active layers. However there has been a drawback that the TFT operating speed is too low to constitute driver circuits.
Meanwhile, the TFT utilizing polysilicon active layers can realize several hundreds to several thousands times the field effect mobility as compared with the amorphous silicon used TFT. This has made it possible to form a high performance driver circuit on a common substrate to a matrix circuit.
Thus the active matrix display devices could be realized by gaining the material of polysilicon. At the present the marketplace has grown to an extent that they are built, as image display monitors, in electronic appliances, such as video cameras and notebook personal computers.
However, demands are raised toward higher precision of image displays as the active matrix display is put into general home use. Moreover, there is also a demand for devising in order to realize higher performance active matrix displays. It is becoming important as a means for achieving this to enhance the driveability for the driver circuit.
It is basically important to reduce the off current (the drain current when the TFT is off) for TFTs formed as pixels for the matrix circuit, wherein the requirement is low for the mobility. On the contrary, the driver circuit TFTs require extremely high field effect mobility for processing a great deal of information signals.
For example, the digital broadcast video signals are transmitted at high frequency, e.g., several tens of MHz (e.g., 80 MHz). On the circuit side, even if signal division or the like is carried out, the shift register circuit must be driven at 10 to 20 MHz. Such high frequency drive requires a field effect mobility of at least 200 cm2/Vs or higher.
Due to these demands, attempts have been made in various ways in order to increase the TFT mobility. There is, among them, a report on a technology to use silicon germanium (Si1-XGeX: where 0<X<1) for the active layer.
Silicon germanium which is narrow in energy band gap as compared to that of silicon can form a channel region with high carrier density. This results in an advantage to provide higher field effect mobility, as compared to the case using silicon for the active layer.
However, there encounters a problem that the off current be increased correspondingly to the increase of carrier density, as compared to the TFT with silicon. Due to this, this technology is not applicable for forming pixel matrix circuits as mentioned before, thus involving a factor to decelerate the applicability to the active matrix display devices.